A polyacetal resin is excellent in mechanical property, fatigue resistance, friction-or abrasion resistance, chemical resistance, and moldability. Therefore, the polyacetal resin has been widely utilized in various fields such as an automotive part, an electric or electronic device part, other precision machinery part, an architectural or pipeline part, a household utensil or cosmetic article part, or a medical device part. However, along with expansion or diversification in application, a polyacetal resin having higher quality has been demanded.
Characteristics (or properties) required for the polyacetal resin include characteristics that mechanical strength in a process step such as an extruding step or a molding step is not deteriorated, that deposit to a metal mold (or mold deposit) is not generated, that mechanical property under a long-term heating condition (heat aging) is adversely affected, and that incomplete (or defective) molding such as silver streak or void is not found in a molded product. As one of the important factors responsible for such deterioration of strength or physical properties, and incomplete molding, the degradation of the polymer upon heating is exemplified. In particular, the polyacetal resin inherently tends to be easily decomposed in an oxidative atmosphere at an elevated temperature or in an acidic or alkaline environment because of its chemical structure. Therefore, the essential need that must be fulfilled for a polyacetal resin includes high thermal stability and minimal emission (or generation) of formaldehyde in the course of processing or from molded products. Formaldehyde is chemically active and ready to be oxidized to formic acid to adversely affect the heat resistance of resin. In addition, when the resin is used as electric or electronic device parts, formaldehyde causes corrosion in metallic contacts or discoloration of the parts due to deposits of an organic compound, and contact errors occur. Furthermore, formaldehyde itself pollutes the working environment in parts assembling as well as the living environment around use of end products.
In order to stabilize chemically active terminals, the following methods are known: for a homopolymer, a method of esterifying the terminal of the polymer by acetylation or other means; and for a copolymer, a method of copolymerizing trioxane and a monomer having an adjacent carbon bond (e.g., a cyclic ether or a cyclic formal) upon polymerization, and then decomposing and removing unstable terminal sites to make the unstable terminal sites stable (or inactive) terminal sites. However, in a heating process, cleavage (or fission) decomposition also occurs in the main chain part of the polymer. Only the above-mentioned treatment is insufficient to prevent the polymer from such a decomposition, and practically, it is considered that addition of a stabilizer (e.g., an antioxidant, and other stabilizers) is essential for such inhibition.
However, even in the case blending these stabilizers, it is difficult to completely inhibit decomposition (or degradation) of the polyacetal resin. In practice, upon melt processing in an extruding step or a molding step for preparing a composition, the polyacetal resin undergoes an action of heat or oxygen inside of a cylinder of an extruder or a molding machine, thereby generating formaldehyde from a decomposed main chain thereof or an insufficiently stabilized terminal thereof, as a result, working environment is worsen in an extruding and molding process. Moreover, in the case carrying out molding for a long period, a finely powdered substance or a tar-like substance is deposited on a metal mold (mold deposit), thereby decreasing working efficiency. In addition, the mold deposit is one of the ultimate factors for deteriorating the surface condition of the molded product. Further, the polymer decomposition causes deterioration in mechanical strength of the resin, and discoloration of the resin. From such a viewpoint, a good deal of effort is continued for establishing more effective stabilizing formulation (or recipe) about the polyacetal resin.
As the antioxidant added to the polyacetal resin, a phenol (or phenolic) compound having steric hindrance (hindered phenol), and an amine compound having steric hindrance (hindered amine) have been known. As other stabilizers, melamine, an alkali metal hydroxide, an alkaline earth metal hydroxide, and an organic or inorganic acid salt have been known. Moreover, antioxidants are generally used in combination with other stabilizers. However, even when such an additive(s) is/are used, it is difficult to avoid formaldehyde emission (or generation) from a molded product of the polyacetal resin.
U.S. Pat. No. 3,152,101 (Patent Document 1) discloses a composition comprising a polyacetal copolymer and a dicarboxylic acid dihydrazide (e.g., an aliphatic dicarboxylic acid dihydrazide having a carbon number of 3 to 10). Although use of such an aliphatic carboxylic acid hydrazide improves heat stability at some level thereby inhibiting emission of formaldehyde, such a composition is low in formability (or moldability). Therefore, mold deposit occurs, or the aliphatic carboxylic acid hydrazide bleeds out of a molded product formed with the composition.
[Patent Document 1] U.S. Pat. No. 3,152,101 (the first and the third columns)